Integrin receptors facilitate immune responses by promoting efficient trafficking of lymphocytes into tissue, and by mediating stable contacts between antigen-reactive lymphocytes and antigen-presenting cells (APCs). On T lymphocytes, the functional activity of integrins, such as LFA-1 (alphaLabeta2), is rapidly enhanced by stimulation of the antigen-specific T cell receptor (TCR). An array of intracellular proteins coordinate TCR signaling to integrins via undefined mechanisms. In the previous funding period, we demonstrated a critical function for the intracellular adapter protein ADAP (adhesion and degranulation-promoting adapter protein) in regulating TCR-mediated activation of LFA-1. TCR stimulation of T cells from ADAP-/- mice fails to enhance LFA-1 functional activity and LFA-1 clustering. ADAP-/- T cells also have an impaired ability to proliferate and produce IL-2 in response to TCR stimulation, despite normal activation of proximal TCR signaling events. In this competing renewal application, we propose to test the hypothesis that ADAP is a multifunctional adapter protein that regulates TCR-mediated cytoskeletal reorganization events critical to localization of LFA-1 and other proteins to the contact site between T cells and APCs. In Aim 1, we will identify the motifs within ADAP critical for regulation of LFA-1 function and clustering. We will utilize a novel adenoviral-based system for expressing ADAP in TCR transgenic ADAP-/- primary T cells expressing human adenovirus receptor. In Aim 2, we will define the mechanism by which ADAP controls cytoskeletal changes critical for LFA-1 function, with a specific focus on Ena/VASP proteins and the Rapl GTPase. In Aim 3, we will expand on preliminary findings that ADAP-/- T cells exhibit defects in TCR-mediated degradation of I?Ba by elucidating the function of ADAP in controlling localization of PKCtheta and other NF-kappaB regulatory proteins to the T-APC contact site. In Aim 4, we will use adoptive transfer systems to define the function of ADAP in regulating T cell responses to antigen in an intact animal. These studies will provide important new insights into ADAP function and integrin regulation critical to our understanding of how T cells mediate an effective immune response to foreign pathogens.